The type of hydraulic power unit shown in the referenced patents has lifting arms which are raised or extended by means of hydraulic power whenever an associated jack stand is to be either raised or extended. The necessary hydraulic pressure is generated manually, by use of a hand pump. The power unit also includes a manually operated valve for dissipating the hydraulic pressure.
The above-referenced copending patent application Ser. No. 350,111 filed May 9, 1989 describes in some detail the mechanical and hydraulic steps that are taken in order to control an extendible jack stand from a separate hydraulic power unit. When a load is being lifted by the jack stand, if by chance the weight of that load exceeds the design capacity of the jack stand, then any excess fluid pressure is automatically relieved and the fluid returned to the fluid source. Thus, a valve with specifically designed release pressure is required to release that excess pressure. When the jack stand has been raised and the power unit is to be detached it is then necessary, by manual control, to unload the entire hydraulic pressure in order to lower the lifting arms. For this purpose an unloading valve is required.
Thus in a system of the foregoing type the control of the hydraulic pressure involves several separate and discrete steps.
During the raising or extension process the lifting arms of the power unit engage a load-supporting plate of the jack stand. Any hydraulic pressure developed in excess of design capacity is automatically released. When the jack stand has been raised or extended by a desired distance it then becomes locked in position by its own mechanism. Specifically, ratchet teeth carried by one telescoping member engage spring-loaded dogs carried by ratchet arms on another telescoping member. The hydraulic pressure utilized for powering the lifting arms may then be unloaded so that the lifting arms may be disengaged from the load-supporting plate of the jack stand and retracted.
Whenever the associated jack stand is to be lowered or retracted, the power unit is placed in alignment with it and hydraulic pressure is generated in order to raise or extend the lifting arms. The lifting arms engage the load-supporting plate of the jack stand in order to transfer its load from the ratchet teeth. The jack stand mechanism is then automatically retracted utilizing a mechanism of the type disclosed and claimed in U.S. Pat. No. 4,697,788. The hydraulic pressure in the power unit is then dissipated, preferably at a controlled rate in order to lower or retract the load at a controlled rate.